chore: add msrv in Cargo manifests

[jlizen]

📬 Issue #, if available:

Relates to #986 but does not close it

✍️ Description of changes:

Adding the Cargo.toml declaration of our MSRV. Currently it is stated in our README that we support 1.81.0 or later, but not somewhere programmatically accessible. Ref: https://github.com/awslabs/aws-lambda-rust-runtime/blob/main/README.md#supported-rust-versions-msrv

Adding the rust-version field to all our crate manifests. This is nice for error messages, MSRV-aware crates resolution, etc. Ref: https://doc.rust-lang.org/cargo/reference/rust-version.html

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• I confirm that I've ran cargo +nightly fmt.
• I confirm that I've ran cargo clippy --fix.
• I confirm that I've made a best effort attempt to update all relevant documentation.
• I confirm that my contribution is made under the terms of the Apache 2.0 license.